Reusable surface insulation containing polybenzazole

ABSTRACT

A lightweight, flexible, layered insulation composed of glass or ceramic fabric covered with a ceramic coating and overlying an insulating felt made from needled polybenzazole (PBZ) material which is preferably polybenzoxazole (PBO), and which optionally contains a poly(1,3-phenylene isophtalamide) felt material, commercially known as Nomex™, either combined with the PBZ felt or layered beneath the PBZ felt as a separate layer. The insulation is readily applied to a reusable launch vehicle via a silicone adhesive.

FIELD OF THE INVENTION

The invention relates to a reusable thermal insulation for use with athermal protection system of a reusable launch vehicle. Moreparticularly, the invention relates to flexible thermal insulation whichmay be applied to the surface of a reusable launch vehicle.

BACKGROUND OF THE INVENTION

The Shuttle Orbiter, the only operational Reusable Launch Vehicle (RLV),is protected during ascent and reentry by lightweight, low thermalconductivity rigid and flexible thermal protection systems (TPS). TheShuttle Orbiter currently uses various thermal protection systems tomitigate aerothermal heating encountered during ascent/reentry. At hightemperatures (up to 1500° F.) quilted ceramic blankets are used forthermal protection. At extreme temperatures, (up to 3000° F.) rigidceramic materials such as porous silica tile and carbon-carbon materialsprovide protection against thermal bum-through. For low temperature uses(up to 750° F.) felt batting systems such as the Flexible ReusableSurface Insulation (FRSI) system are used. The FRSI system consists ofNomex™ batting needled into a large felt-type pad/sheet and coated witha protective silicone topcoat. The coated pad is used on the ShuttleOrbiter in areas that have limited thermal requirements, i.e., areasthat have relatively low aerothermal heating (up to 750° F.). Theadvantage of the FRSI system is that it can be easily installed in largepart sizes onto the vehicle because of its flexible needled-feltconstruction.

The robust nature, simple design, and conformability of the current FRSIsystem make it well suited for extensive use since it can easilywithstand acoustic loading and provide a smooth continuous aerodynamicsurface. However, the inherent material properties of the Nomex™ battingand silicone coating that compose FRSI limit the temperature capabilityof this product to areas that remain below 750° F. such as the topsurface of the fuselage and the upper surfaces of the wings. Thislimitation in thermal stability is unfortunate since its simplicity indesign, low cost, low maintenance, and ease in installation make FRSI anexcellent candidate for more extensive use if the upper temperaturelimit were raised.

While other non-FRSI thermal protection systems are used at present tomanage the thermal requirements over the higher temperature areas of theOrbiter vehicle during ascent and reentry, these other systems are moreexpensive relative to FRSI in terms of installation, maintenance, andreplacement. Furthermore, the simplicity in design of FRSI allows it tobe easily cut to accommodate any size or shape, whereas other thermalprotection systems must be custom fabricated, which results in highermanufacturing costs. What is needed is an insulating material exhibitingthe ease of manufacture and ease of installation associated with FRSIwhile exhibiting improved thermal insulating characteristics.

SUMMARY OF THE INVENTION

The invention is a layered insulation composed of glass or ceramicfabric covered with a ceramic coating overlying an insulating felt madefrom needled polybenzazole (PBZ) material which is preferablypolybenzoxazole (PBO), and which optionally contains apoly(1,3-phenylene isophtalamide) felt material, commercially known asNomex™ fiber, either combined with the PBZ felt or layered beneath thePBZ felt as a separate layer.

The outer mold line (OML) of the insulation is composed of protectiveceramic fabric covered with a ceramic coating material designed towithstand elevated temperatures. The ceramic protective layer ofhigh-temperature glass or ceramic fabric and the ceramic coating provideexceptional thermal protection and efficiently reduce the back-face ortransmitted temperature to the underlying PBZ felt.

The PBZ felt is made up of PBZ fibers that are entangled in a needlingprocess to form a cohesive felt. The needling process providesstructural support via entanglements of the PBZ batting fibers formechanical peel strength. The PBZ felt itself can withstand highertemperatures than the Nomex™ felt used in previous launch vehicleinsulation, while maintaining similar mechanical and thermal properties.Heating is reduced through the PBZ felt as heat migrates from the OMLtoward the (inner mold line) IML. If a Nomex™ felt layer underlies thePBZ layer, then the insulation is designed such that heat transmittedthrough the PBZ layer will have been reduced to correspond with thethermal capacity of the Nomex™ layer. If only a PBZ felt layer is used,the insulation is designed such that the heat is reduced to correspondwith the thermal capacity of the structure of the vehicle.

The separate layers of the insulation are needle stitched together toform a cohesive structure. The needling process uses barbed needles thatare pushed through the fibrous batting, forcing fibers to entanglewithin and between the layers of material, thus “stitching” through theentire construction. The outer ceramic or glass fabric isneedle-stitched to the PBZ batting, thus entangling PBZ felt fiberswithin the woven fabric yams. For increased durability, the ceramiccoating may be used as a bonding agent to mechanically lock the glassfabric and the PBZ batting fibers together. Furthermore, the ceramiccoating acts as an effective thermal barrier to thermally insulate thePBZ batting against hot gas penetration, and provide added protectionfrom other penetrating environmental conditions.

The insulation provides improved thermal protection over previouslightweight flexible insulations, such as FRSI. Thus, the insulation iscapable of providing thermal protection at temperatures exceeding the750° F. limit of FRSI. The insulation provides improved insulatingcharacteristics, yet the insulation may be installed on reusable launchvehicles according to currently established methods for FRSIinstallation, thereby being simpler to install than the other thermalprotection systems.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a side cutaway view of an embodiment of the inventedinsulation,

FIG. 2 is a side cutaway view of insulating felt according to anembodiment of the invention,

FIG. 3 is a side cutaway view of insulating felt according to analternative embodiment of the invention, and

FIG. 4 is a side/top/bottom view of a Space Shuttle Orbiter indicatingproposed regions for use of the invented insulation.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fillyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

Referring now to FIG. 1, the invented insulation 10 comprises a felt pad12 of polybenzazole fiber or a combination of polybenzazole and Nomex™fibers, which is laminated to a coated ceramic fabric layer 13 along theouter mold line surface of the felt 12. The insulation 10 may beattached to the surface of a vehicle 20, such as the Space Shuttle, viaa layer of adhesive 11 between the inner mold line surface of the feltand the surface of the vehicle.

The polybenzazole or combination of polybenzazole and Nomex™ fibers aretemperature resistant, flexible filaments randomly oriented and closelyarranged with respect to each other and are needled to form a needledfelt configuration similar to previous FRSI insulation materials. Thefelt pad 12 is between 0.14 inch and 0.40 inch in thickness, dependingon the insulative capacity needed for a particular application. The unitweights for felt ranging from 0.144 inch to 0.176 inch in thickness arefrom 14.5 oz/yd² to 22.5 oz/yd², and for felt ranging from 0.30 inch to0.34 in thickness are from 27.5 oz/yd² to 37.5 oz/yd².

The term “polybenzazole fiber” as used herein refers to various fibersmade of polybenzazole (PBZ) polymers. Examples of the polybenzazole(PBZ) polymer include polybenzoxazole (PBO) and polybenzothiazole (PBT)homopolymers, as well as random, sequential or block copolymers of theirmonomer components.

The polybenzoxazole and polybenzothiazole, as well as random, sequentialor block copolymers of their monomer components, are disclosed in, forexample, U.S. Pat. Nos. 4,703,103; 4,533,692; 4,533,724; 4,533,693;4,359,567; and 4,578,432. The PBZ polymers are lyotropic liquid crystalpolymers which are composed of homopolymers or copolymers containing, asthe main base unit, at least one selected from the units depicted by thestructural formulas (a) to (h):

The PBZ polymers and copolymers can be produced by any of the knownmethods, such as disclosed in U.S. Pat. No. 5,089,591. The PBZ polymersand copolymers may be made into polybenzazole fibers with hightemperature resistance, high tensile strength, and high tensile modulusby known methods such as that shown in U.S. Pat. No. 5,294,390.

A preferred PBZ fiber for use in the felt 12 is PBO, and a particularlypreferred fiber for use in the felt 12 is a PBO fiber cut to a length of25 to 100 mm. Because PBO is a particularly preferred fiber for use inthe insulation, PBO is used throughout the specification for exemplarypurposes, though it is noted that the described process and compositionsare equally applicable to PBZ fibers, in general.

In addition to PBZ fibers, the felt layer 12 may comprise Nomex™ fiber,a poly(1,3-phenylene iso phthalamide) commercially available from theDuPont Company. The Nomex™ fiber is needled into a felt material as inFlexible Reusable Surface Insulation (FRSI) as described in U.S. Pat.No. 4,151,800. Referring to FIG. 2, the PBO 30 and Nomex™ fiber 32 maybe combined such that the different fibers form a gradient from Nomex™32 at the inner mold line face of the felt 12 to PBO 30 at the outermold line face of the felt 12. Referring to FIG. 3, the PBO 30 andNomex™ fiber 32 may be combined such that the different fibers formlayers, with the Nomex™ 32 positioned adjacent the inner mold linesurface of the felt and the PBO 30 positioned adjacent the outer moldline surface of the felt. If the PBO 30 and Nomex™ 32 layers of the feltare produced separately, the PBO and Nomex™ may be combined by needlingthe two layers together.

As used herein, “needling” is meant as the process of repeatedlyprojecting one or more barbed needles through a material such thatfibers of the material, or multiple materials being needled together,are forced to entangle, thus creating a unitary felt type body fromsingle component materials and thus effectively stitching togethermultiple component materials.

The felt 12 is pre-heat treated prior to installation upon a vehicle byfirst exposing the felt to incremental heat treatments which aregradually increased to about 750° F. This eliminates subsequent adverseshrinkage and allows any volatile materials to be driven off.

A layer 13 of coated ceramic fabric is laminated to the outer mold linesurface of the felt 12. The coated fabric 13 reradiates heat away fromthe felt material and provides a smooth aerodynamic surface.

The coated fabric layer 13 is constructed of a base fabric 14 of wovenceramic fibers. The layer 13 preferably has a thickness of between about0.01 and about 0.06 inches, and most preferably about 0.03 inches. Thefibers of the fabric are ceramic and remain physically stable whenexposed to extreme temperatures, such as those experienced by aspacecraft upon re-entry into the atmosphere. The fibers are continuous,meaning that most of the fibers span a substantial portion of either thelength or width of the woven fabric. Exemplary fabrics are quartz wovenfabrics and Nextel™ fabrics. Of the Nextel™ fabrics, Nextel™ 610,Nextel™ 720, and Nextel™ 440 fabrics are preferred, with Nextel™ 440being particularly preferred due to its lower cost. Quartz woven fabricsare preferably provided with an aluminosilane binder finish. The fiberdimensions of the fabric 14 are not particularly limited, although afiber diameter of from 3 to 15 μm can generally be employed.

A protective ceramic coating 15 is applied to the fabric layer 14. Thecoating is preferably applied to the fabric layer 14 in accordance withU.S. Pat. No. 5,296,288, incorporated herein by reference, whichdescribes the application of an admixture of powder SiO₂, colloidalSiO₂, and an emittance agent.

The method of coating the fiber includes the use of an SiO₂ powdercomponent which is commercially available, such as 99.9% SiO₂, 325 mesh,from Cerac Corporation, Milwaukee, Wis. The colloidal SiO₂ component ofthe protective coating is a suspension of colloidal SiO₂ particles inwater, such as that commercially available as Ludox AS, from du PontCompany, Wilmington, Del. Alumina powder and colloidal alumina may alsobe used. The emittance agent for use in this invention is selected fromthe group consisting of silicon tetraboride, silicon hexaboride, siliconcarbide, molybdenum disilicide, tungsten disilicide and zirconiumdiboride. The emittance agent for use in this invention preferably is inthe form of a powder having a particle size of from 4 to 7 μm. Siliconhexaboride is preferred and an exemplary silicon hexaboride is 98%silicon hexaboride, SiB₆, 200 mesh, from Cerac Corporation, Milwaukee,Wis.

An exemplary fiber coating contains SiO₂ powder in an amount of from23.0 to 44.0 wt %, and preferably from 29.0 to 39.0 wt %, colloidal SiO₂in an amount of from 25.0 to 45.0 wt %, and preferably from 29.0 to 40.0wt %, silicon hexaboride in an amount of from 0.5 to 4.5 wt %, andpreferably from 1.5 to 3.5 wt %, water in an amount of from 19.0 to 39.0wt %, and preferably from 23.0 to 35.0 wt %. The content of eachcomponent is given in terms of the total weight of the protectivecoating.

The protective coating 15 is prepared by first forming a slurry of thecomponents of the protective coating, and then ball milling the slurryto provide a uniform solid dispersion. The slurry is then placed in anappropriate storage container (e.g., pint or quart plastic bottle) androtated on a Kendall or equivalent mixer until just prior to applicationonto the fabric 14.

The protective ceramic coating 15 is applied to the ceramic fabric 14,preferably by use of a spray gun. The coated ceramic material ispreferably uniformly coated such that all filaments, yams and threads ofthe ceramic material are completely covered. A dry coating weight ofabout 0.02 g/cm² is especially preferred. The surface thickness of thedry coating is preferably from 0.08 to 0.012 mm, and preferably has auniformity (standard deviation/average thickness) of +/−10%.

The coated fabric layer 13 is needle-stitched to the PBO felt, thusentangling the PBO felt fibers within the woven fabric yarns. Forincreased durability, the ceramic coating is used as a bonding agent tomechanically lock the ceramic fabric and the PBO felt together. If theceramic coating is to be used as an adhesive, then ceramic fabric 14 isneedled to the PBO felt prior to application of the ceramic coating 15or after application of the ceramic coating to the ceramic fabric butprior to drying of the coating material. The ceramic coating istypically cured at room temperature for about 4 hours. If multiple coatsare used, then the lower layer is typically cured for 4 hours and thetop coat is cured for 8 hours or longer at room temperature.

Referring again to FIG. 1, the finished insulation 10 is bonded to theframe of a vehicle 20 with adhesive 11 and cured. The adhesive bondingagent is one such as RTV-560™, which is a silicone rubber compound madeby the General Electric Company and has heat resistant characteristicsup to 600° F. The thickness of the adhesive 11 is approximately 0.019cm. The curing is preferably in a vacuum bag for not less than 16 hoursat 1.5 to 2.5 pounds per square inch and room temperature.

The invented insulation is flexible and is easily applied to a vehicle.Referring to FIG. 4, the insulation is designed for installation uponthe surfaces 40 of a vehicle which experience relatively lowtemperatures, up to about 925° F. The insulation may be manufactured invarious size and thickness, resulting in an insulation which iseconomical to produce and to install. Since the PBO felt material isheat resistant to a temperature of 925° F., it acts to reduce thetemperature through the thickness of the insulation either to anadditional Nomex™ layer, which is heat resistant to a temperature of731° F., or to the vehicle structure, which is heat resistant to atemperature of less than 400° F. The exterior coated ceramic fabriclayer of the insulation provides a protective shield for the felt. Thespaces between individual fibers of the ceramic fabric allow venting ofthe insulation.

Many modifications and other embodiments of the invention will come tomind to one skilled in the art to which this invention pertains havingthe benefit of the teachings presented in the foregoing descriptions andthe associated drawings. Therefore, it is to be understood that theinvention is not to be limited to the specific embodiments disclosed andthat modifications and other embodiments are intended to be includedwithin the scope of the appended claims. Although specific terms areemployed herein, they are used in a generic and descriptive sense onlyand not for purposes of limitation.

1. A flexible layered thermal insulation, comprising: a felt comprisedat least partially of needled polybenzazole (PBZ) fibers, such felthaving an outer mold line (OML) surface and an inner mold line (IML)surface; a layer of fabric comprised of ceramic fibers, affixed to theOML surface of the felt; and, a ceramic coating applied to the fibers ofthe fabric layer, wherein the felt comprises PBZ fibers in combinationwith fibers of poly(1,3-phenylene isophtalamide) and wherein the felthas a higher percentage of PBZ fibers in the region adjacent the OMLsurface of the felt than the region adjacent the IML surface of thefelt.
 2. The insulation of claim 1, wherein the PBZ fiber is selectcdfrom polybenzoxazole (PBO) and polybenzothiazole (PBT).
 3. Theinsulation of claim 1, wherein the felt comprises a unitary layer ofneedled fibers, and wherein the composition profile of the felt is agradient from the IML surface to the OML surface.
 4. The insulation ofclaim 1, wherein the felt is constructed of a layer of needledpoly(1,3-phenylene isophtalamide) fibers joined by needling to a layerof needled PBZ fibers.
 5. The insulation of claim 1, wherein the basisweight of the felt is 14.5 oz/yd² to 37.5 oz/yd².
 6. The insulation ofclaim 1, wherein the fabric layer is comprised of fiber selected fromthe group consisting of quartz, 1,3-phenylene isophthalamide, andcombinations thereof.
 7. The insulation of claim 1, wherein the ceramiccoating is derived from an admixture of silica powder, colloidal silica,and an emittance agent.
 8. The insulation of claim 7, wherein theemittance agent is selected from the group consisting of silicontetraboride, silicon hexaboride, silicon carbide, molybdenum disilicide,tungsten disilicide and zirconium diboride.
 9. A reusable thermalinsulation attached to the surface of a vehicle, the insulationcomprising a felt comprised at least partially of needled polybenzazole(PBZ) fibers, such felt having an outer mold line (OML) surface and aninner mold line (IML) surface; a layer of fabric comprised of ceramicfibers, affixed to the OML surface of the felt; and a ceramic coatingapplied to the fibers of the fabric layer; wherein the felt comprisesPBZ fibers in combination with fibers of poly(1,3-phenyleneisophtalamide) and wherein the felt has a higher percentage of PBZfibers in the region adjacent the OML surface of the felt than theregion adjacent the IML surface of the felt; and, wherein the insulationis adhered to the surface of the vehicle with an adhesive bonding agent.10. The insulation attached to a vehicle of claim 9, wherein theadhesive bonding agent is a silicone adhesive.
 11. A flexible layeredthermal insulation, comprising: A felt comprised at least partially ofneedled polybenzazole (PBZ) fibers, such felt having an outer mold line(OML) surface and an inner mold line (IML) surface; a layer of fabriccomprised of ceramic fibers, affixed to the OML surface of the felt; anda ceramic coating applied to the fibers of the fabric layer, wherein thelayer of fabric is affixed to the OML surface of the felt by needlingthe felt and the fabric together thereby entangling the fibers of thefelt with the ceramic fibers of the fabric.
 12. The insulation of claim11, wherein the ceramic coating also forms an adhesive bond between theceramic fibers of the fabric layer and the needled fibers of the felt.13. A reusable thermal insulation attached to the surface of a vehicle,the insulation comprising: a felt comprised at least partially ofneedled polybenzazole (PBZ) fibers, such felt having an outer mold line(OML) surface and an inner mold line (IML) surface; a layer of fabriccomprised of ceramic fibers, affixed to the OML surface of the felt; anda ceramic coating applied to the fibers of the fabric layer, wherein thelayer of fabric is affixed to the OML surface of the felt by needlingthe felt and the fabric together thereby entangling the fibers of thefelt with the ceramic fibers of the fabric; and, wherein the insulationis adhered to the surface of the vehicle with an adhesive bonding agent.14. The insulation attached to a vehicle of claim 13, wherein theadhesive bonding agent is a silicone adhesive.